The present invention relates to a liquid crystal display device adopting ferroelectric liquid crystal capable of detailed displaying, particularly, in a large screen, and a manufacturing method the same.
Conventionally, as a liquid crystal display device, a liquid crystal display device using STN (Super-Twisted Nematic) liquid crystal or TN (Twisted Nematic) liquid crystal has been known. In recent years, as a liquid crystal display device capable of highly detailed and large capacity displaying, a liquid crystal display device using ferroelectric liquid crystal has been getting an attention.
As disclosed in Appl. Phys. Lett. 36(1980) pp. 899-901 (N. A. Clark and S. T. Lagerwall), ferroelectric liquid crystal has desirable-characteristics. such as memory effect, high response, and wide viewing angle.
Further, as in the conventional liquid crystal display device of TN system. or STN system, ferroelectric liquid crystal is capable of highly detailed and large capacity displaying by the simple matrix system wherein two electrode substrates are faced each other. Note that, the electrode substrate has an arrangement wherein scanning electrodes and signal electrodes, respectively made of a transparent conducting film in stripes, are positioned in matrix. on a transparent substrate.
However, in the case of adopting the ferroelectric liquid crystal in the liquid crystal display device of simple matrix system, when a ferroelectric liquid crystal display device having a large screen with a highly detailed image is manufactured by providing the stripe electrodes which are made of only the transparent conducting film, the electrode resistance is increased as the stripe electrodes are extended in the lengthwise direction in accordance with the increased displaying area. As a result, driving problems are generated such as generation of heat, delaying of a signal, and rounding of a signal wave which is applied to the pixel region.
Note that, since the conventional liquid crystal display device of TN system or STN system adopts the multi-plexing driving wherein a high contrast image is formed by scanning of a plurality of frames by application of a periodic driving voltage, a problem of lowering of displaying due to the delaying effect of the applied-voltage is not presented. However, in the ferroelectric liquid crystal display device, since it is required that a high contrast image be formed by scanning of a single frame, the delaying effect of the applied voltage becomes a problem.
For the described reason, in the case of adopting a larger screen in the ferroelectric liquid crystal display device, it has been a conventional practice to adopt a method in which the entire electrode resistance is lowered by providing metal electrodes made of a low resistant metal in the lengthwise direction of the scanning electrodes and the signal electrodes. It is required that the metal electrodes are formed along the stripe transparent electrodes (scanning electrodes or signal electrodes) in the lengthwise direction, and that the metal electrodes are electrically connected to the stripe transparent electrodes.
As a first method for forming such metal electrodes, a method for forming the stripe transparent electrodes on a transparent substrate, and then forming the metal electrodes which are electrically connected to the transparent electrodes is available. Specifically, the following methods are available. (1) As shown in FIG. 8, a method for forming metal electrodes 103 on stripe transparent electrodes 102 formed on a transparent substrate 101, along an edge 102b in the lengthwise direction on the upper surface 102a of the transparent electrodes 102, (2) as shown in FIG. 9, a method for forming the metal electrodes 103 on the stripe transparent electrodes 102 formed on the transparent substrate 101, along the edge 102b in the lengthwise direction on the upper surface 102a of the transparent electrodes 102 such that the metal electrodes 103 protrude from the edge 102b to a side surface 102c of the transparent electrodes 102 (Japanese Unexamined Patent publication No. 280724/1989 (Tokukaihei 1-280724)), and (3) as shown in FIG. 10, a method in which the stripe transparent electrodes 102 formed on the transparent substrate 101 are made contact with the metal electrodes 103 on an insulating film 104 via a through hall 105 provided on the insulating film 104 covering the transparent electrodes 102 (Japanese Unexamined Patent publication No. 280724/1989 (Tokukaihei 1-280724)).
However, in the case of adopting the first method, the metal electrodes 103 project out of the upper surface 102a of the transparent electrodes 102 or the upper surface of the insulating film 104 at least by the thickness of the transparent electrodes 102 or the insulating film 104. Here, in the case where the ferroelectric liquid crystal display device is adopted in a large screen panel, it is required that the metal electrodes 103, as a low resistant conducting film for suppressing delaying of the applied voltage, have a film thickness of not less than 0.1 xcexcm, more preferably not less than 0.4 xcexcm. Thus, the metal electrodes 103 project out of the upper surface of the transparent electrodes 102 or the insulating film 104 by at least 0.1 xcexcm. Further, when adopted in a yet larger screen panel, the film thickness of the metal electrodes 103 is required to be thicker.
Also, in order to realize a surface-stabilized ferroelectric liquid crystal. display device, it is preferable that the gap between the facing electrode substrates is set in a range of substantially 1.0 xcexcm to 3 xcexcm. Thus, when adopted in a larger screen panel, a problem is presented that short-circuiting of the metal electrodes 103 projecting out of the upper surface of the transparent electrodes 102 or the insulating film 104 is likely to occur between the upper and lower electrode substrates facing each other.
Furthermore, since the metal electrodes 103 project out of the surface of the transparent electrodes 102 or the insulating film 104, a step-difference is created where the metal electrodes 103 are provided. This presents a problem that the alignment of the liquid crystal is changed where the step-difference is created, and as a result, uniform displaying is not realized.
In order to solve the problems of the first method, the following second through fourth methods are available.
In the second method, stripe metal electrodes are formed on a transparent substrate, and transparent electrodes are formed thereon so as to be electrically connected to the metal electrodes. As the second method, for example, the following method is available. As shown in FIG. 11, after forming the metal electrodes 103 in stripes on the transparent substrate 101, the transparent electrodes 102 are formed in stripes via the insulating film 104 so that the metal electrodes 103 and the transparent electrodes 102 are made contact with each other via the through hall 105 provided on the insulating film 104 (Japanese Unexamined Patent publication No. 63019/1990 (Tokukaihei 2-63019)). In the case of adopting the second method, compared with the case of adopting the first method, the thickness of the metal electrodes 103 can be made thicker, allowing the electrode resistance to be reduced further.
However, in the second method, it is required to provide a manufacturing step for forming the insulating film 104 between the metal electrode 103 and the transparent electrode 102, and thereafter a step for forming the through hall 105 for connecting the metal electrodes 103 and the transparent electrodes 102 to the insulating film 104. This increases the number of manufacturing steps.
Also, in the case of adopting the second method, a function of a black matrix is given to the metal electrodes 103. When the metal electrodes 103 function as a black matrix, it is required that a region A (meshed region in FIG. 11) facing a spacing between adjacent transparent electrodes 102 be covered with the metal electrodes 103. For this reason, when forming the metal electrodes 103, it is required that the metal electrodes 103 be provided in such a manner that the width of the metal electrodes 103 is wider than the width between adjacent transparent electrodes 102 so as to provide a region-where the transparent electrodes 102 and the metal electrodes 103 overlap via the insulating film 104. Thus, in the second method, although the insulating film 104 is provided between the metal electrodes 103 and the transparent electrodes 102, a problem is presented that the possibility of a leaking current flowing between the transparent electrodes 102 and the adjacent metal electrodes 103 is high.
In the third method, as disclosed in Japanese Unexamined Patent publication No. 76134/1996 (Tokukaihei 8-76134), the stripe metal electrodes are. formed on the transparent substrate, and UV (Ultra Violet light) curable resin is injected into gaps between the pattern of the metal electrodes. Namely, as shown in FIG. 12(a) through FIG. 12(d), after positioning a smooth mold 106 which has been applied with UV curable resin 107 so as to face the metal electrodes 103 with the stripe transparent substrate 101 (see FIG. 12(a)), the UV curable resin 107 is exposed by the UV light from the back surface of the transparent substrate 101 so as to form an insulating film having the same thickness as that of the metal electrodes 103 (see FIG. 12(b). Thereafter, the smooth mold 106 is removed (see FIG. 12(c)), and the transparent electrodes 102 is formed on the surface of a layer composed of the metal electrodes 103 and the UV curable resin 107 (see FIG. 12(d). In this method, since the UV light exposure is carried out after the smooth mold 106 which has been applied with the UV curable resin 107 is combined with the transparent substrate 101 provided with the metal electrodes 103, the smoothness of the insulating film made of the UV curable resin is excellent.
However, in the third method, it is required, in order to prevent bubbles from entering the UV curable resin 107, that the smooth mold 106 be combined with the transparent substrate 101 in a vacuum tank. Further, a driving system is required for combining the smooth mold 106 with the transparent substrate 101. The third method also has a problem that the manufacturing process is complicated because the smooth mold 106 is required to be cleaned every time the smooth mold 106 is used.
In the fourth method, as disclosed in J. Electrochem. Soc.; SOLID-STATE SCIENCE AND TECHNOLOGY August 1988 pp. 2013-2016, the LPD (Liquid Phase Deposition) method is adopted using SiO2 amorphous film. The LPD method employs a solution of silicofluoric acid (H2SiF6: HF), and the chemical equilibrium of the solution is shifted to the side of SiO2 deposition.
As shown in FIG. 13, first, the metal electrodes 103 are deposited on the transparent substrate 101 (see FIG. 13(a)), and the metal electrodes 103 are patterned, and while maintaining the photoresist 108 used in patterning (see FIG. 13(b)), a SiO2 film 109 is deposited on gaps between the pattern of the metal electrodes 103 (see FIG. 13(c)). Thereafter, the photoresist 108 on the metal electrodes 103 is removed (see FIG. 13(d)). In this method, no step-difference and no grooves are created on the surfaces of the metal electrodes 103 and the SiO2film 109.
However, in the fourth method, it is required that the metal electrodes 103 be chemically resistant to hydrofluoric acid, and this sets a limit to the material of the metal electrodes 103. Further, because the deposition rate is notably slow, substantially 300 xc3x85/hour, it takes 30 hours to deposit 1 xcexcm. The fourth method also has a problem that the concentration of the silicofluoric acid solution is required to be carefully watched over when depositing the SiO2 film 109.
It is an object of the present invention to provide a liquid crystal display device capable of uniform displaying with high contrast, which is realized by transparent electrodes having a smooth surface, and a manufacturing method of such a liquid crystal display device.
In order to achieve the above-mentioned object, a first liquid crystal display device in accordance with the present invention including a pair of electrode substrates, each having a substrate and a plurality of transparent electrodes provided in stripes on the substrate, and a liquid crystal layer enclosed in a spacing between the pair of elect-rode substrates includes an insulating film made of hard silicon resin, provided between the substrate and the plurality of transparent electrodes of at least one of the pair of electrode substrates, and a plurality of metal electrodes provided in the insulating film, electrically connected individually to the plurality of transparent electrodes.
With this arrangement, a voltage is applied between the transparent electrodes provided in stripes on each of the pair of electrodes, and the alignment state of liquid crystal molecules is switched in the intersecting regions (pixel region) of the transparent electrodes of one of the pair of electrode substrates and the transparent electrodes of the other of the pair of electrode substrates, thus displaying is carried out.
Here, since the metal electrodes are connected to transparent electrodes of at least one of the pair of electrode substrates, the electrode resistance of the transparent electrodes are greatly reduced, and it is possible to suppress rounding of the waveform of a driving voltage applied to the pixel region and temperature nonuniformity in a cell due to generated heat, thus significantly improving the displaying quality. Also, since the metal electrodes are provided between the transparent electrodes and the substrate corresponding to the transparent electrodes, even when the thickness of the metal electrodes is made thicker in order to realize a lower resistance, no short-circuiting is induced.
The hard silicon resin, due to its skeleton of the siloxane linkage (Sixe2x80x94Oxe2x80x94Si), has desirable heat resistance, weather resistance, and water resistance. Also, because the hard silicon resin has a siloxane molecular arrangement of mostly three-functional units and four-functional units, which resembles the skeleton of the glass, the hard silicon resin has desirable surface hardness and abrasion resistance. Also, the silanol group produced by hydrolysis of alkoxysilane is condensed at low temperature in the presence of a catalyst so as to form a hard film, thus allowing the hard silicon resin to be deposited on the substrate with ease.
Therefore, the hard silicon resin, whose property resembles that of glass, can also be etched by hydrofluoric acid as in the glass substrate, allowing the grooves of the metal electrodes to be formed with ease. This allows the use of the manufacturing method wherein grooves are formed on the silicon resin by etching using as a mask photosensitive resin such as photoresist, and the grooves are filled with metals by vacuum deposition or spattering so as to form the metal electrodes by the lift-off method, thereby obtaining with ease a liquid crystal display device capable of uniform displaying with high contrast.
A second liquid crystal display device of the present invention including a pair of electrode substrates, each having a substrate and a plurality of transparent electrodes provided in stripes on the substrate, and a liquid crystal layer enclosed in a spacing between the pair of electrode substrates, includes a first insulating film and a second insulating film respectively provided between the substrate and the plurality of transparent electrodes of at least one of the pair of electrode substrates, the first insulating film being made of an insulating material which is not etched by dry etching using oxygen plasma or oxygen ion, the second insulating film being made of light-transmissive resin, and a plurality of metal electrodes provided in the second insulating film, electrically connected individually to the plurality of transparent electrodes.
With this arrangement, as in the first liquid crystal display device, since the metal electrodes are provided, the electrode resistance of the transparent electrodes are greatly reduced. Also, since the metal electrodes are provided between the transparent electrodes and the substrate corresponding to the transparent electrodes, even when the thickness of the metal electrodes is made thicker in order to realize a lower resistance, no short-circuiting is induced.
The light-transmissive resin can be etched with oxygen plasma or oxygen ion, and allows grooves to be formed with ease for providing the metal electrodes. This allows the use of the manufacturing method wherein grooves are formed on the light-transmissive resin by etching using photosensitive resin such as photoresist as a mask, and the grooves are filled with metals by vacuum deposition or spattering so as to form the metal electrodes by the lift-off method, thereby obtaining with ease a liquid crystal display device capable of uniform displaying with high contrast.
Further, since the first insulating film is made of an insulating material which is not etched by oxygen plasma or oxygen ion, the first insulating film acts as an etching stopper when etching is carried out, and the depth of the grooves is accurately controlled, thus allowing the surface height of the metal electrodes to be controlled.
It is preferable that the first and second liquid crystal display devices have an arrangement wherein (1) the surface height of the metal electrodes formed in the insulating film and (2) the surface height of the insulating film are equal.
It is also preferable that the first and second liquid crystal display devices have an arrangement, in order to realize detailed and large capacity displaying, wherein the liquid crystal layer includes ferroelectric liquid crystal.
In order to achieve the above-mentioned object, a method for manufacturing the liquid crystal display device of the present invention includes the steps of (a) depositing an insulating material on a substrate, (b) subjecting the insulating material to photolithography and an etching process using photosensitive resin so as to form an insulating film in stripes, while maintaining the photosensitive resin on an upper surface of the insulating material, (c) depositing a metal so as to cover the insulating film and the photosensitive resin, (d) removing an excess portion of the metal together with the photosensitive resin while maintaining the metal between stripes of the insulating film so as to form a layer composed of the insulating film and metal electrodes, (e) depositing a transparent conducting material on the layer composed of the insulating film and the metal electrodes, and (f) subjecting the transparent conducting material to photolithography and an etching process using photosensitive resin so as to form transparent electrodes in stripes electrically connected to the metal electrodes.
With this manufacturing method, the insulating film is formed in stripes, and thereafter a metal is deposited on the insulating film by vacuum deposition or spattering without removing the photosensitive resin so as to form the metal electrodes by the liftoff method. Thus, it is ensured that the metal electrodes are implanted in the insulating film without inducing pattern shifting. Further, since the metal is deposited by vacuum deposition or spattering, it is possible to control the film thickness of the metal with ease such that the surface of the metal electrodes and the surface of the insulating film coincide with a step-difference of within 30 nm. Therefore, it is possible to substantially completely eliminate the step-difference on the surface of the transparent electrodes formed on the metal electrodes and the insulating film. As a result, it is possible to manufacture a liquid crystal display device capable of realizing uniform displaying with high contrast without adversely affecting the alignment and switching characteristics of the liquid crystal.
In the above manufacturing method of the liquid crystal display device, it is preferable, in order to ensure that the metal electrodes are implanted by the lift-off method through dry etching using oxygen plasma or oxygen ion, that the insulating material deposited on the substrate is light-transmissive resin.
In the above manufacturing method of the liquid crystal display device, it is also preferable, in order to ensure that the light-transmissive resin is patterned in stripes with ease, that the etching of the insulating material is carried out by dry etching using oxygen plasma or oxygen ion.
In the above manufacturing method of the liquid crystal display device, it is also preferable, in order to ensure that the metal electrodes are implanted by the lift-off method through etching using hydrofluoric acid, that the insulating material is hard silicon resin which is prepared by curing heat curable silicon resin.
In the above manufacturing method of the liquid crystal display device, it is also preferable, in order to ensure that the silicon resin is patterned in stripes with ease, that the etching of the insulating material is carried out by wet etching using hydrofluoric acid.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.